A marrow stem cell genetic defect(s) has been implicated in events leading to loss of B-cell tolerance in systemic lupus erythematosus (SLE). The defect may be limited to B- and/or T-progenitors or, alternatively, affect multiple lineages. A candidate gene for this defect may affect apoptosis, the process of programmed cell death. For example, overexpression of Bcl- 2, a protooncogene inhibiting apoptosis, results in SLE-like disease. A definitive way to evaluate these hypotheses would be to test committed progenitors of various lineages for the presence of the lupus genetic defect in New Zealand Black (NZB) mice. First, we will test the cellular level of the genetic defect using a unique combination of in vitro and in vivo studies. Populations of early B progenitors from NZB and BALB/c mice will be grown in a culture system recently developed in our laboratory. These cells will then be used to reconstitute the B-compartment of B- and T-cell deficient SCID animals. Transfer of normal and autoimmune T-cells will allow direct evaluation of whether NZB B-cells, T-cells or both are required for disease. B-progenitor populations overexpressing Bcl-2 will serve both as positive control and a model for development of autoimmunity. Second, to determine the genetic defect, we will use a cDNA cloning strategy to identify NZB genes involved in apoptosis. Genes will be selected through rescue of cells from apoptosis. Expression studies will identify those genes implicated in autoimmunity. Methods to block gene expression will confirm their role in disease. This work will be important in understanding lymphoid hematopoietic cell growth and death and the processes leading to autoimmune disease.